If you're stuck with using scalars (I'd recommend getting unstuck ASAP and use an actual data structure) then you could do it like this

no strict 'refs';
for($cgi->param) {
  ## &func is bad form, see. perlsub
  complain()
    unless exists  ${__PACKAGE__."::"}{$_}
       and defined *{ ${__PACKAGE__."::"}{$_} }{SCALAR}
       and defined ${ *{ ${__PACKAGE__."::"}{$_} }{SCALAR} };
}
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So for each parameter check if the package variable with the name of the given parameter is defined (check symbol table entry, then type glob entry for SCALAR, then value referenced in type slot) and complain if not. So as you can see, using symbolic variables is a very icky route indeed (as Dominus illustrates very very well).
HTH

_________
broquaint